Commercial equipment for atomizing and electrostatically depositing coating material, such as paint, commonly utilizes either airless or air atomization of the coating material. In coating certain types of articles, as where a high coating delivery rate is desired, or where there is a need to penetrate into a recess, for example, it is desirable to atomize the coating material without the presence of air. This is done by projecting the coating material through a small nozzle orifice under high pressure. The interaction of the pressurized stream of coating material with air as it passes through the small nozzle orifice causes a break-up, or atomization, of the coating material into small particles, which then may be electrostatically charged.
The electrostatic charge has the effect of improving the efficiency of deposition of the coating material onto the substrate being coated. An electrode, also sometimes called an antenna, is commonly located near the spray nozzle, and is connected to a source of high voltage to establish an electrostatic field in the vicinity of the region of atomization. The electrostatic field imparts a charge to the spray particles which causes the particles to be attracted to a grounded substrate. The charged atomized coating material is in effect drawn to the substrate, resulting in increased and more efficient deposition of coating material.
An airless spray gun and spray gun system such as that described is disclosed in U.S. Pat. No. 4,355,764. Spray guns of this type are characterized by an elongated electrode for charging the atomized spray from the gun nozzle. The electrode of such guns is characteristically connected to a high voltage power supply through electrical circuitry contained in the the spray gun body. Such circuitry includes a high ohmage resistor in the gun barrel to reduce the current flow to the electrode and to avoid inadvertent discharge of electricity or arcing if the gun is moved too close to a grounded workpiece or too close to a grounded wall of the spray booth within which the gun is operating.
In order to further reduce the current to the electrode and the capacity of the gun, such guns also characteristically include a "tip" resistor in the electrical circuit between the barrel resistor and the electrode. A typical tip resistor has a bent thin wire lead extending from its forward end to electrically connect the tip resistor to the electrode. Generally this electrical connection includes a conductive washer positioned so that the base of the electrode contacts one face of the washer and the bent wire lead the other face. The bent lead end gives the connection some resiliency to accommodate spacing differences between the electrode and tip resistor due to tolerance variations in the nozzle assembly parts. The rearward end of the tip resistor characteristically has another thin wire lead which connects the tip resistor to the barrel resistor.
It has been found that when the nozzle assembly is removed and then replaced in electrostatic spray guns of the type described hereinabove, such as for changing nozzles or for cleaning of the nozzle assembly, the lead at the front of the tip resistor is bent and flexed. Repeated bending and flexing can result in the lead snapping, thereby breaking contact between the tip resistor and the electrode, and interrupting power to the electrode. The tip resistor then has to be removed from its bore and replaced with a new tip resistor having a good lead.
Replacement of the tip resistor in this type of gun has been complicated by the fact that the rearward lead of the tip resistor has to be connected to the barrel resistor. The rearward lead has to be inserted in the front of the bore in which the barrel resistor is received where it can contact the conductive end of the barrel resistor. In order to ensure good electrical contact however, the barrel resistor has to be removed for proper positioning of the tip resistor lead, with the barrel resistor than replaced in position. This procedure entails further disassembly of the gun in order to access the barrel resistor. A good solid electrical connection between the elements making up the electrical circuitry, and between those elements and the electrode, is of course of critical importance in the operation of the spray gun.
The nozzle assemblies of spray guns of the type described typically have a number of small internal parts. Some of these parts are loose when the assembly is not attached to the gun.
As is often the case when cleaning the nozzle assembly, it will be taken off of the gun over a vat of cleaning solvent. The loose parts can easily fall out of the nozzle assembly and into the solvent vat, where they can be difficult to locate and retrieve. The loose parts can likewise be dropped and become lost when the nozzle assembly is being changed. Replacement of the lost parts is of course costly, and consumes time when the spray gun could otherwise be productively used.